Remove Italian sausage from casings. Crumble and cook. Drain liquid and set aside.
Unfold one sheet of un-thawed puff pastry onto a lightly floured surface. Cut into one-inch rounds with a cookie cutter.
Place rounds on a lightly greased cookie sheet. Grease the bottom of a glass baking dish and place on top of the pastry rounds. Bake for 10-12 minutes, until golden brown. Cool on a different cookie sheet.
Top each round with one teaspoon of heated pasta sauce, three tiny rounds of mozzarella, a sprinkle of Parmesan cheese and Italian sausage.
Pop them back into the oven for a few minutes, until the cheese melts.